The present invention relates to the general field of visualization of materials or surfaces on a monitor by using computer graphic techniques. In general, a digital representation of a real material or surface is rendered, mapped onto a target object of arbitrary shape, and the simulated appearance of the target object is then visualized under user selected illumination conditions and viewing directions.
From the start, it should be understood that the field of this invention is not image creation or manipulation, which typically includes only 2- and 3-dimensions and does not require that the result resemble reality, much less be renderable in reality. The field here is visualization of simulated materials that are renderable in reality—they are physically plausible materials.
Getting an accurate visualization of a simulated appearance is, for most materials and surfaces occurring in the real world, an extremely challenging technological problem. Consequently, much effort has been expended on finding approximations that are both of aesthetically pleasing appearance and quickly computed, albeit without concern that the resultant appearance is representative of a material that is physically plausible, such as required by a product designer.
Further, in creative applications like product design, large databases of materials are required. Various functions have been developed, such as the Bidirectional Texture Function (BTF), which is a material representation fitting a large number of complex materials of many different types. However, to measure a sufficiently representative BTF for a given real material requires relatively complex measuring equipment and the measurements are very time consuming. A representative surface area of the material must be measured pixel by pixel for a large number of illumination directions and viewing directions, typically by using a number of digital color cameras and a complex illumination system with hundreds of spot-type lamps distributed over the hemisphere above the material being measured.
With the BTF measured, the visualization itself, i.e. the graphical representation of the material on the monitor under any desired illumination condition and any desired viewing direction and applied or mapped to any object of any shape, is realized by digital rendering techniques which use the BTF database as input data. Suitable rendering techniques and software are well known in the computer graphics art and are not subject of the present invention. Such rendering techniques or software can retrieve color reflectance values from the BTF database for each given pixel of the real material for each given illumination direction and for each given viewing direction. Intermediate values can be calculated by interpolation from the actual values stored in the BTF database.
Unfortunately, the creation of such databases usually containing thousands of materials requires substantial technical effort both in terms of measurement time and device sophistication and is therefore often not practicable and often prohibitively expensive.
As an alternative to producing databases for a large number of individual materials a set of basis materials with corresponding BTFs might be provided from which the designer can chose a sample and then modify or edit the BTF on-the-fly to meet his requirements. Here, an intuitive and fast editing approach is necessary to maintain efficiency in the creative process. Moreover, the goal of the editing process is to generate physically plausible results that represent simulated materials that may be manufactured.
One common approach is to fit analytical reflectance models to the data (McAllister 2002, Daubed 2001) and to perform a modification of model parameters afterwards. For efficient parameter changes additional methods might be used which simplify propagation of parameters across the material surface (An 2008). While using reflectance models guarantees physical plausibility to a certain degree, manually finding new parameters to match a desired target appearance is a tedious task. Moreover, this approach is limited to materials that can be faithfully described by simple reflectance models. Most complex materials, especially those exhibiting special features like glittering or significant and large surface structures, cannot be reproduced by such an approach with high accuracy.
Other methods do not rely on analytical models but modify reflectance data in a more direct way. Lawrence 2006 used inverse shade trees and an optimization scheme coined ACLS to decompose the spatially varying material properties of planar samples from dense hemispherical samplings in a collection of 1D curves and 2D textures. This approach factorized the material into multiple, low-dimensional parts, which could be edited separately, causing certain reflectance changes on the whole material when reconstructing from the factorized representation. While this approach allows for a high-accuracy representation of the source material, editing of the single parts still remains a manual process and a desired target appearance is therefore difficult to achieve. Moreover, the method is only applicable for flat materials.
In Kautz 2007 a first set of editing operators for generic BTF data was proposed. While those operators can deal with arbitrarily complex materials, their heuristic nature means that the physical plausibility of the final result may be low. Additionally, a given target material is very difficult to match as the operator parameters have to be specified manually.
A combination of the edit propagation algorithm (An 2008) and the editing operators defined in Kautz 2007 was made by Xu 2009. While this simplifies the usage of the edit operators, it does not overcome the basic problems of physical implausibility and manual work.
Using measured reflectance on both the source and target side of the editing process was proposed in An 2011. This algorithm can transfer reflectance data from one material to match the spatial reflectance distribution of a second material allowing for very intuitive editing with minimal manual effort. While the method allows enrichment of sparsely captured representations (even single images) with highly detailed reflectance data, the other way around is not possible as the edited material is always represented by reflectance samples from the target material.
There is a need to easily and quickly develop a synthesized appearance of a simulated material that not only “looks good,” but is physically plausible to enable product designers to evaluate a wide range of simulated materials, knowing that any material selected by the designer from the range of materials is physically plausible for making real world products.